1. Field of the Invention
This invention relates to a heavy duty radial tire having a tread of a rib pattern and more particularly to an improvement in the prevention of irregular wear which occurs on the edges of ribs formed between circumferential grooves.
2. Prior Arts
Generally employed for large vehicles such as trucks and buses are heavy duty radial tires provided with ribs formed between circumferential grooves having zigzag edges for good traction property on a wet surface. Tires of this type are susceptible to irregular wear such as step wear, that is, earlier wear of the outer edges of ribs arranged at the outermost position in the widthwise direction of the tread which occurs earlier than the other portion of the tread and progresses along the outer edges of the ribs as well as toward the center of the tread, or furrow wear, that is, earlier wear of the rib edges which protrude into the grooves and which is easily deformed when in contact with the ground, which occurs In an initial stage of tire travelling and progresses toward the center of the ribs.
It is known that such irregular wear occurs at an initial stage of tread wear and progresses preferentially but such irregular wear does not occur in the course of tread wear if it does not occur at an initial stage of tread wear. There have been made some proposals for the prevention of such irregular wear. U.S. Pat. No. 4,230,512, for example, proposed to prevent the occurrence of step wear by providing narrow circumferential grooves at the slightly inward positions from the outer edges of the outermost ribs. Since furrow wear starts from both zigzag edges of the rib which protrude into the grooves, there have been provided tires having the circumferential grooves made straight to provide the ribs free from zigzag edges or lateral unevenness. However, the tires provided with straight grooves and ribs are deficient in stopping ability on a wet ground, and also have a poor furrow wear prevention effect. There have been also provided tires having a plurality of sipes extending laterally from the rib edges and disposed at short intervals in the circumferential direction along the rib edges, in order to enhance the stopping ability on a wet ground and the prevention of furrow wear. Such tires, however, are not fully convincing.